OSV-2020-2100 Bad-cast to pcpp::IDnsResource from invalid vptr

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=26669 Crash type: Bad-cast Crash state: Bad-cast to pcpp::IDnsResource from invalid vptr pcpp::DnsLayer::parseResources pcpp::DnsLayer::DnsLayer...